FIG. 1 is a block diagram illustrating the structure of a conventional WDM-PON. Referring to FIG. 1, the conventional WDM-PON includes a central office 100, an optical distribution network 142 and optical network units 140-1, 140-2, . . . , and 140-N (hereinafter, commonly designated as 140).
The conventional WDM-PON performs bi-directional communication by using two different wavelength bands. For instance, a downstream signal is transmitted from the central office 100 to the optical network unit 140 through an A wavelength band, such as 1570-1620 nanometers. An upstream signal is transmitted from the optical network unit 140 to the central office 100 through a B wavelength band, such as 1450-1500 nanometers.
The central office 100 includes a plurality of optical transmitters 101, 102 and 103 for transmitting downstream signals having an A wavelength band, a plurality of optical receivers 104, 105 and 106 for receiving upstream signals having a B wavelength band, a plurality of band division filters 107, 108 and 109 for coupling/decoupling light having an A wavelength band to/from light having a B wavelength band, and a 1×N multiplexer/demultiplexer 110, wherein N represents the number of optical network units.
The optical unit network 140 includes a plurality of optical transmitters 118, 119 and 120 for transmitting upstream signals having a B wavelength band, a plurality of optical receivers 115, 116 and 117 for receiving downstream signals having an A wavelength band, and a plurality of band division filters 121, 122 and 123 for coupling/decoupling light having an A wavelength band to/from light having a B wavelength band.
The optical distribution network 142 is located between the central office 100 and the optical network units 140-1 to 140-N and includes a first-branch optical cable 113 for transmitting optical signals having multiplexed wavelengths from the central office 100 to a remote node 150, a 1×N multiplexer/demultiplexer 114 for multiplexing/demultiplexing optical signals transmitted thereto from the central office 100 and the optical network units 140, and N second-branch optical cables 124, 125 and 126 for connecting the 1×N multiplexer/demultiplexer 114 to the optical network units 140. In this regard, the 1×N multiplexer/demultiplexer 114 has a role of a remote node.
Since the optical distribution network 142 of the conventional WDM-PON has a double-branch structure including the first-branch optical cable 113, the second-branch optical cable 124-126 and the remote node 150, if the number of optical network units for the WDM-PON is N, all of the N optical network units 140 are connected to one remote node 150. Thus, the remote node 150 is located at a center of the N optical network units in order to minimize an amount of optical cables (i.e., optical fibers) to be used. However, if it is difficult to install the remote node 150 at the center of the N optical network units 140 due to geographic or local characteristics, the remote node 150 may be connected to the optical network units 140 through an inefficient structure, resulting in waste of expensive optical fibers. In particular, if the conventional WDM-PON having the two branches of optical distribution is applied to agricultural villages in which the optical network units 140 are sporadically distributed over the whole area of agricultural villages, optical fibers are unnecessarily wasted and installation work thereof is difficult.